An apparatus comprising a TEM to which an EDS detector is attached is known. This apparatus permits one to perform an x-ray analysis of a sample while observing an image of this sample. Such apparatus is disclosed in U.S. Pat. No. 4,910,399. FIG. 2 shows the arrangement of the objective lens of this apparatus and the EDS detector. The objective lens comprises a top polepiece 1 and a bottom polepiece 2. A sample holder 3 is disposed between these two polepieces. The EDS detector, indicated by 4, is positioned near the gap between the polepieces 1 and 2. A collimator 6 which determines the illuminating angle of the x-rays entering the radiation-sensitive surface 5 of the detector 4 is disposed between the radiation-sensitive surface 5 and the sample holder 3. A film 7 made of a carbon material is formed on the inner surface of an opening formed in the collimator 6 through which the x-rays pass. The optical axis is indicated by 0.
When an electron beam hits the sample (not shown in FIG. 2), the characteristic x-rays are produced from the sample. At the same time, an electron beam transmitted through the sample, secondary electrons and scattered electrons are produced from the sample. These electrons collide against the sample holder 3, the top surface of the bottom polepiece 2, the tapering portion 9, and the inner surface 8 of a hole formed in the bottom polepiece 2. As a result, x-rays characteristic of the materials of the sample holder 3 and of the bottom polepiece 2 are produced from the holder 3 and polepiece 2. There is a possibility that these characteristic x-rays impinge on the radiation-sensitive surface 5 of the EDS detector 4. In the conventional energy-dispersive x-ray spectrometer of this kind, it is only necessary that the sample holder 3 be made of a light element such as beryllium having a spectrum lying outside the range of wavelengths to which the EDS detector 4 is sensitive. In the prior art TEM, the gap G between the top polepiece 1 and the bottom polepiece 2 is about 10 to 15 mm. Therefore, the thickness T of the holder 3 can be set to about 2.5 mm. Therefore, the holder 3 does not transmit the above described characteristic x-rays emanating from the bottom polepiece 2. As a result, the characteristic x-rays emanating from the bottom polepiece 2 are not detected by the EDS detector 4.
In recent years, however, the resolutions of transmission electron microscopes have been improved greatly. The polepiece gap G is inevitably made as narrow as approximately 2 mm. The thickness T of the sample holder 3 is only about 0.5 to 0.7 mm. In addition, the electron beam density has been increased. The result is that the characteristic x-rays emanating from the bottom polepiece 2 penetrate through the holder 3 and reach the EDS detector 4, where the x-rays are detected. The bottom polepiece 2 is made of an iron-cobalt alloy. Furthermore, the spectra of the characteristic x-rays from Fe and Co are within the range of the wavelengths to which the EDS detector 4 is sensitive. As a result, the output signal from the detector contains a quite high level of background noise. This presents serious problems in performing an x-ray analysis.